Wet air oxidation is a well-known treatment process for the removal of COD and BOD from industrial and municipal wastewater streams. The processes involve contacting a wastewater stream with an oxidizing source, such as oxygen, ammonium nitrate and nitric acid at elevated temperatures and pressures to oxidize pollutants. Most carbonaceous material is converted to carbon dioxide. The nitrogen present either from organo-nitrogen compounds or other sources is converted to nitrogen gas.
The following references illustrate wet oxidation processes:
Proesmans, Luan and Buelow of Los Alamos National Laboratory (Ind. Eng. Chem. Res. 1997, 36 1559-1566) report on a high temperature and pressure (500xc2x0 C./345 bar) hydrothermal oxidation process to remove organic compounds from a waste stream using ammonium nitrate as the oxidizing agent. In the oxidation of methanol and phenol, the authors report that unless an excess of oxidizable carbon is present, NOx in the effluent may become a problem. To avoid NOx production and reduce carbon components to carbon dioxide, a polishing step using hydrogen peroxide is suggested.
GB 1,375,259 discloses the wet oxidation of carbon and nitrogen containing materials to gaseous reaction products using HNO3 and/or a nitrate as oxidizing agent, at temperatures of between 150xc2x0 C. and the critical temperature of water. The preferred oxidizing agent is NH4NO3, which disappears completely from the reaction medium. Example VII shows the treating of a waste stream of caprolactam, the sodium salt of aminocaproic acid and sodium sulfate with nitric acid at a temperature of 300xc2x0 C. at 15 bars. The patentees report that slow heating of the reaction mixture resulted in reduced corrosiveness of the reactant mixture.
U.S. Pat. No. 4,654,149 discloses the use of a noble metal catalyst supported on a titania carrier in a wet oxidation process to decompose ammonium nitrate at 250xc2x0 C. for 60 minutes. Approximately from 50-99% decomposition of both ammonium nitrate and nitrite is achieved without air present. Further examples show wet oxidation of phenol with 0.2 times the required amount of oxygen.
JP 60-98297, JP 61 257,292 and JP 61 257,291, discloses the catalytic wet oxidation of ammonium nitrate wastewaters with 1.0 to 1.5 times the stoichiometric oxygen required for ammonia decomposition, at a pH of 3-11.5 at a temperature from 100 to 370xc2x0 C. with a supported noble metal catalyst.
U.S. Pat. No. 5,118,447 discloses a process for the thermochemical nitrate destruction where an aqueous solution of nitrate or nitrite is contacted with a stoichiometric amount of formic acid or formate salt, depending upon the pH. Wet oxidation is effected by heating to 200 to 600xc2x0 C. in the liquid phase to form elemental nitrogen and carbon dioxide. The reaction may be carried out over a pH range of 0-14.
U.S. Pat. No. 5,221,486 discloses a denitrification process where the types of nitrogen compounds present in a waste stream are identified and quantified. The oxidized and reduced forms of nitrogen are balanced and, then, an appropriate nitrogen containing reactant, such as ammonia or a nitrite or nitrate compound, is added and the mixture is heated to 300 to 600xc2x0 C. under pressure to effect denitrification.
U.S. Pat. No. 5,641,413 discloses the two stage wet oxidation of wastewater containing a carbonaceous and nitrogen species. In the first stage the COD is removed by wet oxidation at a temperature of less than 373xc2x0 C. and a pressure sufficient to maintain a liquid water phase. The remaining nitrogen compounds are converted to nitrogen in the second stage by adding sufficient inorganic nitrogen-containing compound to the oxidized wastewater to produce essentially equal concentrations of ammonia-nitrogen, nitrite-nitrogen plus nitrate-nitrogen and a waste stream of reduced COD. Mineral acid is added to the oxidized wastewater to produce a pH between 4 and 7. Optionally, a transition metal salt is added, to catalyze a thermal denitrification step. The last step is conducted at 100xc2x0 to 300xc2x0 C. to decompose the nitrogen compounds.
D. Leavitt et al in Environmental Progress 9 (4), 222-228 (1990) and in Environ. Sci. Technol. 24 (4), 566-571 (1990) reported that 2,4-dichlorophenoxyacetic acid, atrazine and biphenyl were converted to CO2 and other non-harmful gases (N2 and N2O) trough the homogeneous liquid phase oxidation with ammonium nitrate. These reactions were carried out by dissolving the substrates in polyphosphoric acid, adding ammonium nitrate and then heating to about 260xc2x0 C. for some period of time. Although this clearly shows that ammonium nitrate is a good oxidizing agent, it is not a process lending itself to treating aqueous waste streams containing only 1,000 to 10,000 ppm TOC.
This invention relates to an improvement in a thermal process for the removal of organic carbon and organic or inorganic nitrogen-containing pollutants from wastewater streams using nitrate salts as the denitrifying agent. The improvement resides in using nitrate salts of aromatic amines (such as those present in the toluenediamine (TDA) waste water) as the main denitrifying source for the dinitrotoluene (DNT) wastewater. To reduce the corrosiveness of the treated effluents, the pH of the liquid is maintained in the range of 1.5 to 8 and preferably from 1.5 to 4 by adjustment with alkali metal.
This process can offer several advantages including:
an ability to work with an influent wastewater having a low pH (1.5 to 2.5) without causing significant corrosion;
an ability to eliminate organic carbon and organic or inorganic nitrogen-containing pollutants in a single step;
an ability to work under strong acidic conditions, e.g., high sulfate, and achieve excellent nitrate removal;
an ability to reduce nitrate and ammonia levels to drinking water standards when working under almost redox balanced conditions in a short period of time (20 minutes);
an ability to reduce nitrate and ammonia levels to drinking water standards when working under almost redox balanced conditions in a short period of time (20 minutes) and in the presence of a relatively high sulfate or phosphate concentrations; and,
an ability to convert most of the organic carbon and organic or inorganic nitrogen-containing pollutants into carbon dioxide and nitrogen gas despite the fact that the operational NH4+/NO4xe2x88x92 is much smaller than that required by the stoichiometry (1.66).
In our co-pending applications, Ser. No. 09/613,206 having a filing date of Jul. 10, 2000 and Ser. No. 09/659,055 having a filing date of Sep. 11, 2000, the subject mater of each being incorporated by reference, improvements for reducing the corrosiveness of waste streams contaminated with sulfur or phosphorous containing compounds, whether organic or inorganic, while maintaining reaction rate were proposed. The first application proposed: operating said process within a pH range from about 1.5 to 8 and preferably within a pH range of from about 1.5 to 4 by appropriate addition of alkali metal. The second application proposed adding organic material to the waste stream to provide acetate ion in a molar ratio from 0.06 to 0.17 moles per mole nitrate or, in the alternative, should the waste stream contain organic material convertible to acetate in the wet oxidation process, maintain a level of organic material sufficient to provide acetate ion in an amount of at least 0.06 moles per mole of nitrate. The addition, or maintenance of organic material convertible to acetate ion acts as a corrosion inhibitor or buffer assisting in reducing corrosion at pH values of 4 and lower.
However, in the above processes the presence of strong acid anions, nitrate ion removal by ammonium ion becomes more difficult because these acids inhibit the denitrification process. Thus, if a high percent nitrate removal is desired (as expected for effluents that are going to be discharged into the environment), then excess reducing agent or longer reaction times may be required. These two alternatives, i.e., addition of alkali and longer reaction times have some disadvantages, i.e., it may require more than one step or require the use of larger reactors.
This improvement to the above processes uses nitrate salts of aliphatic and aromatic amines to remove nitrate from waste streams and to reduce total organic content (TOC). Thus, denitrification and TOC removal can be accomplished in a single step.
Thus, nitrate and nitrite, including TOC, can be removed from a waste stream by thermal treatment with salts of aliphatic and aromatic amines such as toluene-diammonium nitrate salts or the aniline nitrate salts. The use of aromatic or aliphatic amines has the advantage that they can act as better denitrifying agents than ammonia when used in the presence of sulfate or phosphate. The toluene-diammonium nitrate salts are shown; 
During the process, the nitrate salts of aromatic amines, such as, TDA salts will decompose to give mainly carbon dioxide and nitrogen gas according to the following chemical equation: 
Thus, to achieve removal of carbonaceous and nitrogenous compounds to a desirable level the oxidation and reduction properties of all oxidizable and reducible species present in the wastewater stream are balanced. Because some of the oxidizable species may decompose to gaseous products (i.e. methane, ethylene, etc) that do not participate in the nitrate removal process, balancing the waste stream does not necessarily means stoichiometric balance of the influent wastewater but rather a small excess of reducing agent may be needed as shown in some of the illustrative examples.
Examples of nitrate salts of aliphatic amines and aromatic amines in addition to those mentioned include C1-4 alkyl amines, e.g., methyl amine, ethyl amine, butyl amine; cycloaliphatic amines such as cyclohexane diamine, cyclohexylamine, methylcyclohexyl amine and methylcyclohexyldiamine.
As shown in the above chemical equation, the degradation of nitrate to nitrogen gas by the nitrate salt of TDA requires that acid be consumed acid during the process. Therefore, as the reaction progresses, addition of acid to the reaction media may be necessary to ensure acceptable reaction rates.
Methods for the treatment of wastewater streams containing nitrate, nitrite, sulfate and organic pollutants with ammonia have been previously described. In these processes, ammonium ion (i.e., from ammonium sulfate) acts as the major reducing agent reacting with nitrate (or nitrite) ion to form mainly nitrogen. Organic carbon, typically present in small amounts, would be almost fully oxidized to carbon dioxide during the process.
In a continuous process, a convenient way to provide acidity for this process is by adding some minor amounts of ammonium sulfate (2.2 g/l to 4.7 g/l) to produce acidity as shown in the equation below:
xe2x80x832.5(NH4)2SO4+3NaNO3xe2x86x924N2+9H2O+0.5Na2SO4+2HNaSO4
Another alternative is to provide the acidity by feeding the reactor with sulfuric acid as in a semi-batch process.
Because maximum nitrate removal requires working under strong acidic conditions, corrosion of the metal container will occur. However, there are two contributing factors that prevent the corrosion of the stainless steel reactor. One of them consists on balancing or partially balancing anions from strong acids with alkali or alkali earth metal cations. The other comes from the corrosion inhibition effect provided by some organic by-products generated during the oxidative degradation of the organic reducing agents. These features are shown in the prior mentioned copending applications. In particular, nitrate salts of toluenediamines decompose with the formation of acetic acid that acts as a corrosion inhibitor allowing us some flexibility when optimizing the M+/SO42xe2x88x92 ratios.
The first step in accomplishing removal of carbonaceous and nitrogenous components to a desirable level requires balancing the oxidation and reduction properties of all of the oxidizable and reducible species present in the wastewater stream. All nitrogen containing species, organic or inorganic, produce substantially only nitrogen and minor amounts of nitrous oxide gas and all carbon containing species produce substantially only carbon dioxide.
One key to pH control in the first step, and to the maintaining of reaction rate during wet oxidation of wastewater streams contaminated with sulfur or phosphorus substances and alkali and alkaline earth metals (designated M), is in the control of the M/SO4xe2x88x922 and M/PO4xe2x88x923 ratio (equivalence basis). This is accomplished as follows: contaminants whose anions are of strong acids, e.g., sulfate and phosphates are balanced with alkali or alkaline earth metal cations and conversely, cations of strong bases are balanced with sulfate or phosphate. The ratio of M/SO4xe2x88x922 is maintained from 0.1 to 4, preferably 0.2 to 1, most preferably from 0.4 to 0.7 and the ratio of M/PO4xe2x88x923 of from 0.1 to 2, preferably 0.2 to 0.67 during wet oxidation. Lower ratios,  less than 0.4 for M/SO4xe2x88x922 may be tolerated when the process effluent designed permits operation with some residual carbon compounds in the effluent. High ratios reduce reaction rate.
The second step in the process involves the balancing of organic species such that on substantial reduction of nitrogen in the wet oxidation process there remains sufficient carbonaceous material in solution under the process conditions in the form of e.g., a) acetic acid and/or its derivatives such as esters, amides, salts, etc; or b) carbonaceous compounds that upon oxidation are precursors to acetic acid or its derivatives. Typically, the molar ratio of acetate to nitrate is kept from falling below 0.06:1.
Pressures are controlled to a high enough pressure to maintain a liquid phase behavior for both the influent and the effluent. If gas phase conditions occur, the salts in the wastewater oxidation product may precipitate and cause plugging of the reactor.